Reference associated type in trait bounds

Question

I have a trait which is meant to tighten constraints on another trait, eg:

trait AssocA {}
trait AssocB: AssocA {}
trait A { type MyAssoc: AssocA; }
trait B: A { type MyAssoc: AssocB; }

If I were using generics rather than associated types, I'd be able to tell Rust that MyAssoc is the same across traits A and B :

trait AssocA {}
trait AssocB: AssocA {}
trait A<MyAssoc> where MyAssoc: AssocA {}
trait B<MyAssoc>: A<MyAssoc> where MyAssoc: AssocB { }

How can I do the same with associated types?

Answer 1

You can refer to the implementing type via Self and since B: A , Self::MyAssoc already exists.

trait B: A where Self::MyAssoc : AssocB {}

This prohibits impl B for T {} when <T as A>::MyAssoc does not implement AssocB . (example)